Data values are often provided at the intersections of coordinates of one type with coordinates of another type, e.g., the intersections of radial coordinates with arc coordinates, or the intersections of x coordinates with y coordinates. If the data values are reliable, they can be used to form an image; but if they are not reliable, as where there is a low signal-to-noise ratio, where there are "drop-outs" when no data value is provided, or where there are singular or odd data values for various reasons, an image formed from these actual data values can have a very poor quality. Instead of using the raw data values at the coordinate intersections, it has been proposed that a display data value be derived for each intersection by using an arithmetic average of adjacent data values that may or may not be weighted; but this does not solve the problem because one maverick data value can have a great effect on the average.
A situation of the type referred to is encountered in the formation of two-dimensional maps of the turbulence and velocity of blood flow in the circulation system of a patient because the signal-to-noise ratio is poor and because of implementation errors of some apparatus that produces no signal at all if the signals are too weak. Furthermore, the velocity signals are really estimates derived from Doppler shift frequencies, and since they are usually sampled, aliasing may occur so that what is actually a large positive velocity can appear as a large negative velocity. Thus, if positive velocity is illustrated by one color and negative velocity by another, small areas of one color may appear in large areas of the other. As such changes in velocity are not likely to occur, the image is not a true representation of the action taking place.